With recent advances of electronic technologies, there is an accelerating proliferation of portable electronic equipment such as mobile phones, notebook personal computers, audio/visual devices, and mobile terminal devices. The portable electronic equipment is usable by a secondary battery. New types of secondary batteries have been introduced. With the aim of developing smaller and lighter batteries and those with a higher energy density, the secondary batteries have been evolved from sealed lead batteries to nickel-cadmium cells, nickel-hydrogen cells, and lithium ion cells. For any type of the secondary batteries, cell active materials and high-capacity battery structures are developed to increase energy density and efforts are taken to realize longer life batteries.
Meanwhile, efforts have been made to further reduce the power consumption of portable electronic devices by improving the functions of the devices. However, it is expected that the total power consumption of a portable device will tend to increase in the future, because new functions should be added to provide improved services to meet user needs. Therefore, this will lead to a need for a power supply with a higher energy density, that is, a longer life power supply enabling longer continuous use.
Fuel cells have lately attracted attention as such a power supply. The fuel cells have a property of a maximum power point. The maximum power point is defined as a power point where the output power of a fuel cell changes from increasing to decreasing when the amount of an output current from the cell exceeds a certain threshold value. Therefore, a range of use of fuel cells must be limited up to the maximum power point. For example, for a Polymer Membrane Fuel Cell (PEFC) which is a fuel cell using hydrogen as a fuel, a protective system has been proposed (e.g., Japanese Patent Application Laid-Open No. 2003-229138).
Recently, a Direct Methanol fuel Cell (DMFC) is expected as a power supply usable in mobile devices under a lower temperature than the PEFC. FIG. 1 shows current-voltage (I-V) characteristics of PEFC and DMFC and a current-power (I-P) characteristic of DMFC. As the characteristics of DMFC, a value of no-load voltage (hereinafter referred to as OCV) may be 0.8 V or above, which is similar to the OCV of PEFC. However, according to a typical voltage-current characteristic of a single DMFC cell, a usable voltage region as actual operating power is 0.4 V and below, and the maximum power point is around 0.2 V. In the DMFC characteristics, the voltage of usable area is remarkably smaller than that of PFFC. Therefore, there is a distinct difference between DMFC and PEFC in characteristics, though both DMFC and PEFC are called fuel cells. It is difficult to directly apply a control practice for PEFC to DMFC, and a control practice specialized for DMFC is required.
The DMFC characteristics greatly vary depending on a working temperature and the flow rate (humidity) of gas at air electrodes. Moreover, the DMFC output power may rapidly decrease for some cause (such as clogging of reaction by-products such as carbon dioxide or water). A minimum requirement for a DMFC used as a power supply is that it is able to supply power required for equipment in which it is used. However, it is supposed that the DMFC may become unable to supply required power according to the condition of the fuel cell as described above. Consequently, a new system and a control method are required.
In a state where a fuel cell cannot supply required power, in particular, the following phenomenon can occur: too much current is drawn from the fuel cell, far exceeding the maximum power point. Drawing excessive current, if occurs, causes problematic conditions such as fuel cell deterioration by polarity inversion, fuel cell temperature rise, and deteriorating performance due to by-products such as carbon dioxide or water. The fuel cell must be controlled adaptively to each of these conditions. However, this poses problems in which state prediction of a high-speed fuel cell is very difficult and provision of a great number of sensors for prediction is costly.
To address the above problems, an object of the present invention is to provide a power supply apparatus and a method of controlling thereof, enabling maximum power point tracking control adaptive to fuel cell conditions by efficient provision of sensors.